The present invention relates to spacer blocks positioned between aluminum ingots, more particularly, the treatment of aluminum ingots and/or spacer blocks to prevent production of oxide staining and sticking during heating of stacked aluminum ingots.
Preheating of aluminum alloy ingots is a well established practice for achieving desired properties in the ingot and to render the ingot sufficiently malleable for reduction and other processes. During the preheating step, aluminum ingots are heated to temperatures below the melting point of the aluminum alloy, e.g., up to about 620xc2x0 C. At these temperatures, alkali metals and alkaline earth metals (e.g., magnesium) in the aluminum alloy migrate to the surface of the ingot and react with available oxygen to form a layer of an oxide (e.g., magnesium oxide) on the ingot. A magnesium oxide layer has a dark color of brown to gray. When an ingot having this dark surface is subsequently rolled, the dark layer becomes a streak of dark color in the rolled product. For certain applications of the rolled sheet, such dark streaks are unacceptable in the marketplace for cosmetic reasons.
A common method of reducing the production of magnesium oxide is to operate the preheat furnace in an atmosphere of vaporized ammonium fluoroborate. The ammonium fluoroborate reacts with magnesium on the surface of the ingots preferentially over oxygen and also possibly facilitates hydrogen loss from the ingot which would otherwise form bubbles in the ingot. In a pusher furnace in which all surfaces of the ingots therein are exposed to the ammonium fluoroborate atmosphere, the surfaces of the ingot uniformly remain as a shiny silver color. However, many preheat furnaces require that the ingots are stacked within the furnace. In order to expose as much surface area of the stacked ingots as possible to the furnace atmosphere the stacked ingots are spaced apart in a stack by a plurality of spacer blocks. Spacer blocks have been made from aluminum alloys and ceramics such as calcium silicate. Other suitable materials for spacer blocks include titanium alloys, steel alloys, and nickel alloys. Drawbacks to aluminum spacer blocks include their propensity to stick to the ingot undergoing heating, indent the ingot and produce unwanted debris on the ingot. Ceramic spacer blocks avoid these problems associated with aluminum spacer blocks. However, staining of the ingot persists on the surface of the ingot which was in contact with the spacer block during heating. Ceramic spacer blocks prevent the atmosphere of ammonium fluoroborate from reaching the interfaces between the spacer blocks and the ingots. Distinct areas of dark magnesium oxide stain are produced on the ingots at the spacer block interfaces. The stained areas may be removed by scalping the ingot, however, this produces more waste, lowers the recovery values and adds another expensive and time consuming step.
Accordingly, a need remains for a method of preheating stacked aluminum ingots maintained spaced apart from each other by spacer blocks which avoids production of dark stains above and below the spacer blocks.
This need is met by the method of the present invention of treating a surface of a stacked aluminum alloy ingot during a heating process having the steps of a) positioning a spacer member adjacent an aluminum alloy ingot to form a stack, wherein the spacer member includes a support surface contacting a contact surface of the aluminum alloy ingot, and at least one of the support surface and the contact surface includes a fluorine containing material; and b) heating the stack to at least a temperature at which the fluorine containing material decomposes such that a layer of a fluorinated oxide compound forms on the contact surface. The fluorinated oxide formed on the contact surface preferably is a fluoride and/or oxyfluoride of an alkali metal fluoride and/or an alkaline earth metal. In general, the support surface includes the fluorine containing material but the contact surface of the ingot or both may include the fluorine containing material.
The spacer member may be made from aluminum alloys, ceramics, titanium alloys, steel alloys or nickel alloys or combinations thereof. Preferred materials for the spacer block are ceramics, ceramic composites and metal laminated ceramics.
The fluorine containing material may be an organic or inorganic fluorine containing material. Suitable inorganic fluorine containing materials include aluminum fluoride, aluminum bifluoride, ammonium fluoroborate, ammonium fluoride, calcium fluoride, sodium aluminum fluoride, magnesium fluoride, magnesium hexafluorosilicate, potassium fluoride, sodium fluoride, and sodium hexafluorosilicate. Suitable organic fluorine containing materials include polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene, tetrafluoroethylene-perfluoro(alkylvinyl ether), tetrafluoroethylene-ethylene, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene, vinylidene fluoride-1-H-pentafluoropropylene, polyvinyl fluoride, tetrafluoroethylene-perfluoroethylene sulfonic acid, fluorinated ethylene propylene (e.g., tetrafluoroethylene-propylene), ethylene-chlorotrifluoroethylene and perfluoroalkoxy copolymers.
The fluorine containing material preferably decomposes or vaporizes at about 200-660xc2x0 C. and preferably is potassium fluoride or polytetrafluoroethylene. The method of the present invention is particularly suited for treating alloys of the Aluminum Association 1XXX, 2XXX, 3XXX, 4XXX, 5XXX, 6XXX, 7XXX or 8XXX series.
The method further includes an initial step of applying a treating composition including the fluorine containing material onto the support surface or the contact surface by brushing, spraying, dipping, or rolling. The treating composition may further include a solvent, hinder, surfactant or dispersant.
The present invention further includes a spacer member having a support surface for contacting an aluminum ingot, the support surface including a treating composition including a fluorine containing material.